I. Field of the Invention
The present invention relates generally to gas generating propulsion systems and principally those suitable for use in rocket motors or for firing artillery pieces. More particularly, the invention is directed to an improved system for creating high surface area for burning in an uncatalyzed or partially catalyzed bulk propellant mass at high pressure and velocity using a highly efficient separating device.
II. Related Art
The successful firing of rocket motors and large caliber munition cartridges rests in no small measure with the performance of the associated propellants including the repeatable predictability of that performance. Important aspects include loading density and burning rate. However, the two commonly seem to work against each other because dense loads inherently create difficulties in achieving a sufficiently rapid and high, increasing ignition surface area necessary for successful high performance burning.
With respect to ammunition, efforts have been directed toward increasing the amount of propellant available per unit cartridge volume (propellant charge density) without sacrificing burn rate by employing a variety of perforated grain shapes. Bulk liquid propellants have also been used, but have generally exhibited unpredictable burn characteristics.
FIG. 1 depicts a typical large caliber round which may be fired from the main turret cannon of a tank or other large caliber device and having propellant loaded in accordance with one prior art method. The round is shown generally at 10 at FIG. 1 and includes a base plate section 12 connected with the wall of a cartridge casing and having a generally cylindrical portion 14 and a necked down or tapered upper portion 16. The cartridge shell itself is normally made of metal or a combustible material such as molded nitrocellulose or other such material which is consumed during the firing of the shell. The projectile itself is shown at 18 with discarding sabot members 20 and 22 which peel away and drop off just after the projectile is discharged from the muzzle of the cannon. A plurality of stabilizing guidance fins as at 24 are also provided. The nose cone section 26 may contain an electronics package and the warhead section 28 may contain arming and detonating circuitry.
With respect to the firing of the shell, a primer housing shown generally at 30 contains a conductive ignition electrode or primer button (not shown). The primer housing is connected with a generally hollow brass or other type metal primer tube 32 which has a plurality of openings as at 34 which access and address the general propellant charge volume 36. The available propellant charge volume is filled with closely packed, generally uniformly shaped granular solid propellant grains 38 which may be 2 to 3 cm long by about 0.5 cm in diameter.
The shell is normally fired electrically using direct current to ignite the primer in the primer housing and through the primer tube 32, thereby igniting the main propellant 38 via the openings 34. In accordance with improving one aspect of performance, i.e., achieving the highest, repeatable muzzle velocity for the projectile, it is desirable that the propellant burn as rapidly and uniformly (with respect to the load) as possible.
Other propellant configurations have included extruded stick shapes. The propellant manufacture begins with carpet rolled propellant, which is dried, aged, pre-cut for extrusion, extruded with perforations and cut to length. Each length is blended to minimize lot to lot performance variation, and each length must be notched or kerf cut in several places on the side to prevent over pressurization during the burn before the propellant may be used. The loading process for a cartridge using stick propellant is labor intensive and the stick shapes have presented difficulties in achieving high loading densities. Performance is not optimum because of mating surfaces of the sticks, as in the case of random placement with granular propellant.
In addition, repeatability of acceptable or good performance of stick propellant also requires uniformity of the notch or kerf size and web between the kerfs for proper burning. Current extrusion and kerf cutting processes are rarely able to achieve this so that the sticks must be blended or mixed prior to loading to achieve some uniformity. As a result of mixing the stick propellant, performance is not optimized.
Another method utilizing ribbed sheet propellant rolled into cylindrical sections has been tested on smaller caliber ammunition. This method used longitudinal ribs replacing perforations to assist ignition. The rolled method experienced difficulty in conformance to the projectile geometry, poor progressitivity, poor flame spread and poor ignition characteristics.
Additional load arrangements for solid propellants are shown and described in Kassuelke et al (U.S. Pat. No. 5,712,445) assigned to the same assignee as the present application. Those loadings are generally in the form of arrangements of closely packed perforated slab or disk-shaped.
There remains a need for a propulsion system that improves pressure control while delivering a high burn rate from a relatively dense load. The present invention represents a different approach to allowing increased loading density in a dynamic system that enables a high, controlled burn rate. Embodiments of the system of the present invention are adaptable to both rocket motor and projectile-firing uses.
In accordance with desirable artillery performance, it is desirable that the pressure history in a launch tube be held nearly constant. This is especially important with respect to higher burning efficiency configurations. An additional goal of manufacture, particularly relevant to cartridge, is to reduce production costs related to shaping grains and labor costs related to loading the shaped grains into the casings. A propellant system which allows increased and more reproducible burning together with lower production and loading costs is very desirable.
Accordingly, it is a primary object of the present invention to provide a system that creates a high surface area propellant at high pressure and velocity from a bulk propellant mass which achieves a controlled burn.
Another object of the invention is to provide an improved propellant system at a lower production cost.
A further object of the present invention is to provide a propellant system wherein a bulk mass of uncatalyzed or partially catalyzed propellant is pressed through a shredder plate with many holes or orifices to create a large number of long, high surface area strings, which simultaneously burn to produce a very high burn rate propellant.
A still further object of the present invention is to create such a system wherein the pressure to force the propellant through the perforated plate can be programmed to suit various design conditions.
A yet further object of the present invention is to provide a system wherein the uncatalyzed or partially catalyzed propellant can itself be used as the pressurization source by suitably adjusting the area of the pressure face on the chamber side versus the bulk propellant side.
A yet still further object of the present invention is to provide a system that provides auto ignition of the propellant strings by a combination of pump work heating and friction between the extended propellant strings and the rims of the holes.
Yet another object of the present invention is to provide a system that can be operated in a stable mode wherein variations and pressure and burn rate are self-correcting to a design rate.
A yet still further object of the present invention is to provide such a system that can be operated in a gun application wherein the system is in an inherently unstable mode in which the pressure tries to increase arbitrarily, but is controlled by an initial shaped volume of inert material that limits the pressure rise until the rate of cavity enlargement is appropriate to the inherent instability of the burn rate that the pressure history can be held nearly constant.
Other objects and advantages will become apparent to those skilled in the art upon familiarization with the specification, drawings and claims contained herein.
By means of the present invention, a propellant concept is provided in which high burn surface area is generated during the burn by a pressure operated shredding device. The shredding may be accomplished by a rotating cutter device or a highly perforated orifice plate. Relative motion between the propellant grain and the shredding device forces the propellant through the device orifice openings or into cutter blades at high velocity and pressure during the burn to generate a large surface area of propellant from a grain that can be a bulk propellant mass of uncatalyzed or partially catalyzed propellant material having the general consistency of a heavy paste.
The shredder engine concept of the invention using the perforated orifice plate is exemplified by, but not limited to, two basic configurations. They include inherently stable or self-regulating and inherently unstable system configurations. The orifice plate itself is preferably of a spherical shape to maximize shape stability and minimize necessary thickness and pressure loss. The system can be designed to provide auto ignition derived from heat generated by the pressurization of the propellant and the friction between propellant and rims of the orifice openings by making the orifice plate from a material having relatively low thermal conductivity thereby allowing additional heat buildup. In one inherently stable configuration, a highly perforated stationary orifice member is located toward the nozzle or outlet of a rocket motor case or the projectile-containing end of a shell cartridge. A similarly shaped, possibly congruent, and preferably hollow piston member is located at the opposite end and partially encloses or is adjacent to a small charge and an igniter. The orifice and piston are separated by the main propellant charge. The burning of the main propellant charge occurs beyond the orifice plate in a region called the chamber, fixed in size in the case of a rocket motor nozzle, but ever-lengthening with movement of the projectile in the case of a gun.
In operation, this system is essentially self-regulating to a design pressure particularly in the case of a rocket motor in which the chamber volume is fixed. The charge behind the piston has a burn rate that is generally fixed by its surface geometry. The dynamics of the system remain stable because:
(1) if the pressure in the chamber drops, the xcex94p between the chamber and the pusher charge will increase, causing the flow rate of the propellant to increase and, in turn, increase the chamber pressure;
(2) if the chamber pressure increases above the design level, the xcex94p between the chamber and the pusher charge decreases, causing the propellant flow rate into the chamber to decrease, which, in turn, decreases the chamber pressure.
In essence, the system hunts to find its design level. This type of system is ideally suited to rocket applications where constant thrust is desired.
In a second configuration, the orifice plate moves away from the projectile or rocket motor nozzle and through the main propellant grain. The system is ignited using a small ignition charge at the projectile or nozzle side of the orifice plate of the system. In this embodiment, the highly perforated orifice plate is carried by a slender central stem or rod which is designed to collapse as the shredder orifice moves aft.
The xcex94p needed to pump the propellant through the orifice plate in this case is governed by the ratio of the orifice stem area to the chamber area. The force on both sides of the orifice plate must be equal so that:
pcAc=Pp(Acxe2x88x92As)
where subscript xe2x80x9cpxe2x80x9d denotes conditions in the propellant and subscript xe2x80x9csxe2x80x9d denotes the stem. Given the above, it will be recognized that the pumping action is purely a function of the geometry. No pusher charge is needed and the ignitor charge has ignition as its sole function providing pressure and flame to get the process started. In one sense, then, the second orifice system is simpler and more efficient that the first, however, the second system is not inherently stable. As will be explained, this condition makes the system well suited to an application where instability may fit the requirement perfectly as is the case with a gun-launched projectile.
In a conventional powder operated gun, the powder burns almost all at once. The pressure rises in a spike and then as the bullet gathers speed and distance the pressure decays rapidly. Modern gun propulsion technology is concerned with extending the peak pressure over time such that the integrated work is maximized and the peak pressure is reduced. This is also the effect that the embodiment of the present invention achieves, particularly if an inert start-up wedge of shreddable material is used about the outer periphery of the orifice plate. The pressure can then be tailored to be approximately level and the peak broadened considerably. The function of the inert start-up wedge is to allow the propellant feed into the chamber to be progressive in rate as the distance and velocity of the projectile both increase. The shape of the pressure curve is, thereby, modulated by the shape of the inert wedge and advantage is gained from the configuration""s tendency toward ever higher feed rates.
Another embodiment of the propellant system of the present invention includes a multi-edge or multi-blade, high speed, rotating shredder screen which pulls a cylindrical or annular tube of solid propellant through the blades and into a fixed nozzled combustion chamber which contains an annular start-up grain and a set of radially mounted spin vanes. Bore riding obturators are provided for and aft to increase efficiency. This embodiment is particularly well suited for use in a recoilless gun arrangement.